PROJECT SUMMARY Corneal transplantation is the most common solid organ transplantation worldwide, and immunological graft rejection is the main cause of graft failure. Corticosteroid eye drops must be applied frequently to prevent graft rejection and rescue grafts that show early signs of rejection, because topically administered drugs undergo rapid clearance and have poor ocular bioavailability. However, long-term, frequent dosing is associated with low patient compliance, and those that comply have risk of increased intraocular pressure (IOP), which can cause glaucoma. We propose a new method for providing sustained corticosteroid delivery at low, but efficacious levels, removing the treatment burden from patients and improving the safety of corticosteroids. We developed biodegradable nanoparticles containing high loadings of dexamethasone sodium phosphate (DSP) that can be administered by the same route that surgeons administer DSP at the end of corneal transplant surgery, subconjunctival (SCT) injection. Further, DSP-loaded nanoparticles (DSP-NP) are coated with polyethylene glycol (PEG) to minimize inflammatory reactions in the eye, and in contrast to topical drops, do not cause an increase in IOP in animal models. Our preliminary results demonstrate that DSP-NP are effective for both preventing and treating early signs of graft rejection in a rat model. Here, our aim is to develop longer- lasting formulations that can safely and effectively prevent and treat corneal graft rejection, for as long as 3 months with a single injection. If successful, this approach would significantly improve corneal transplant patient care and prognosis. In Aim 1, we will confirm batch reproducibility, determine the maximum tolerated dose (MTD), and assess ocular pharmacokinetics (PK). The two lead formulations will then be tested for dose dependent efficacy for prevention and treatment in our rat model of graft rejection in Aim 2. Finally, we will carry out detailed safety and pharmacokinetics studies of at least one lead DSP-NP formulation in rabbits in Aim 3 to facilitate future development and potential clinical translation.